1. Field of the Invention
The present invention relates to a thin film transistor substrate, especially, a thin film transistor substrate of, for example a liquid crystal display device of active matrix type using thin film transistors, a liquid crystal display panel and a liquid crystal display device.
2. Description of Related Art
In a liquid crystal display device of active matrix type, non-linear elements (e.g., switching elements) are disposed in a manner to correspond to a plurality of pixel electrodes arranged in matrix, respectively. The liquid crystal in each pixel is always driven, in principle, (at a duty ratio of 1.0). Therefore, in comparison with the so-called "simple matrix type" which employs a time division driving system, the active system has better contrast and has become an indispensable technique particularly in a color liquid crystal display device. A typical example of the switching element is a thin film transistor (TFT).
In a conventional thin film transistor substrate of a liquid crystal display device of active matrix type, pure Al, Al--Pd or Al--Si is used as a material of a scanning signal line and a gate electrode, and an anodized oxide layer of Al is formed on the scanning signal line and gate electrode.
Here, the liquid crystal display device of active matrix type using the thin film transistors is known in JP-A-2-85826 or on pp. 193 to 210 of Nikkei Electronics entitled "Active Matrix Type Color Liquid Crystal Display of 12.5 Type Adopting Redundant Construction" and issued on Dec. 15, 1986 by NIKKEI McGRAW-HILL, for example.
Available as prior arts are U.S. Pat. No. 5,132,820 (Someya et al), U.S. Pat. No. 4,786,148 (Sekimura et al) and JP-A-62-269120. None of them, however, give a description "a scanning signal line and a gate electrode are made of an alloy of Al and an insulating film formed of an anodized oxide film of the scanning signal line or the gate electrode is formed on the surface of at least one of the scanning signal line and the gate electrode".
U.S. Pat. No. 5,028,122 (Hamada et al) and JP-A-2-106723 (Nagase) resemble the present invention in point of a description "a gate electrode is made of Ta and an insulating film formed of an anodized oxide film of the gate electrode is formed on the surface of the gate electrode" but fail to give a description "an alloy of Al and Ta is used in a gate electrode to provide a higher insulating property of the anodized oxide film than that obtained when Ta is used". In the present invention one of the features is represented in the description that "a scanning signal line and a gate electrode are made of an Al and Ta alloy and an anodized oxide film of the scanning signal line or the gate electrode is formed on the surface of at least one of the scanning signal line and the gate electrode" to thereby obtain a higher anodized oxide film insulating characteristic than that obtained with Ta, as shown in FIG. 17.
In a thin film transistor substrate using pure Al as a material of a scanning signal line and a gate electrode, a hillock takes place; in a thin film transistor substrate using Al--Pd as a material of a scanning signal line and a gate electrode, a hillock takes place and besides an anodized oxide film has a low breakdown voltage; and in a thin film transistor substrate using Al--Si as a material of a scanning signal line and a gate electrode, a residue is generated after etching. Accordingly, in any of the above thin film transistor substrates, the fabrication yield and reliability are degraded and the production process becomes complicated for the purpose of improving the fabrication yield, thus raising the cost of production.